There are storage systems, which comprise multiple physical storage devices (hereinafter, PDEV) comprising a RAID (Redundant Arrays of Inexpensive Disks) group.
In addition to either one or multiple RAID groups, this type of storage system generally comprises a controller (hereinafter called “storage controller”) for accessing each PDEV comprising the RAID group. The storage controller, for example, receives an I/O (Input/Output) request (for example, either a read request or a write request) from a higher-level apparatus (for example, a host computer or another storage system), and, in accordance with this I/O request, sends an I/O command (for example, a read command or a write command) to a PDEV comprising the RAID group. Hereinafter, a request such as an I/O request received by the storage controller will be called a “request”, and a request such as the I/O request received by the PDEV will be called a “command”.
The PDEV may be a flash memory drive. For example, a NAND-type flash memory (hereinafter, a NAND-type flash memory will simply be called a flash memory in the explanation here) is mounted in the flash memory drive. The flash memory is a write-once type in which a data overwrite is not possible, and possesses features such as a data read and write being performed in units of pages, and a data erase being performed in units of blocks comprising multiple pages.
The storage system disclosed in Patent Literature 1 is known as a storage system in which the PDEV is a flash memory drive. This storage system performs a correction read in a case where data is to be read from one flash memory drive (hereinafter, the “target drive” in this paragraph) for which destaging (in which data is stored from a cache memory) is possible from among the multiple flash memory drives comprising the RAID group. In a correction read, multiple pieces of data (any one of which may be parity) is read from each of the multiple flash memory drives other than the target drive in the RAID group, and data is restored inside the target drive based on these pieces of data.